1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor and a method for producing the same.
2. Description of the Related Art
Conventionally, in an electrophotographic image forming process in an electrophotographic application apparatus such as copiers and laser printers, gas lasers having a comparatively short wavelength such as He—Ne lasers, Ar lasers, and He—Cd lasers have been used as light to which a surface of an electrophotographic photoreceptor is exposed so as to form electrostatic latent images. CdS, ZnO, Se or the like, which forms a thick layer, has been used for a photosensitive layer of an electrophotographic photoreceptor that can be used with such a gas laser. Therefore, light for exposure with which the electrophotographic photoreceptor is irradiated by the gas laser is completely absorbed by the thick photosensitive layer, so that interference caused by reflection on the substrate surface of the electrophotographic photoreceptor did not occur.
In recent years, instead of the gas lasers, semiconductor lasers or light-emitting diodes (abbreviated as “LED”) that are compact and inexpensive have been increasingly used as a light source to which an electrophotographic photoreceptor is exposed. With the transition of the light source to be used, electrophotographic photoreceptors having photosensitivity to light having a long wavelength of 700 nm or more that is emitted from the semiconductor lasers or LEDs have been used. For example, multilayered electrophotographic photoreceptors having a multilayered structure in which a charge generating layer comprising a phthalocyanine pigment such as copper phthalocyanine or aluminum chloride phthalocyanine and a charge conveying layer are laminated have been used.
When the electrophotographic photoreceptors having photosensitivity to light having a long wavelength are mounted in an electrophotographic printer of a laser beam scanning system and is exposed to a laser beam, non-uniformity in the images with a pattern of interference fringes may occur in the formed images. The non-uniformity in the images with a pattern of interference fringes occurs partly because the laser light having a long wavelength is not completely absorbed by the photosensitive layer, and the light transmitted through the photosensitive layer reaches the substrate surface of the electrophotographic photoreceptor and is reflected. Then, the reflected light is multiple-reflected in the photosensitive layer and thus becomes a coherent light, resulting in interference fringes.
One approach to prevent such interference fringes that cause non-uniformity in the images is to produce roughness on the substrate surface of the electrophotographic photoreceptor. FIGS. 15A and 15B are views showing the manner in which light is reflected on a substrate surface. FIG. 15A shows the manner in which light is reflected on a smooth substrate surface 1. Incident light beams L11, L12 and L13 are reflected regularly on the smooth substrate surface 1. Since the thickness T1 of a photosensitive layer 2 formed on the smooth substrate surface 1 is formed uniformly, the light beams L11, L12, and L13 reflected on the substrate surface 1 are also reflected regularly on the surface of a photosensitive layer 2. Therefore, in the case where the substrate surface 1 is smooth, the light beams L11, L12, and L13 having a matched phase are multiple-reflected and are mutually intensified (weakened) so as to form an interference pattern. Thus, interference fringes also occur in images formed on the surface of the photoreceptor.
FIG. 15B shows the manner in which light is reflected on a rough substrate surface 3. On the rough substrate surface 3, incident light beams L21, L22 and L23 are reflected irregularly and scattered in different directions from each other. The thickness of a photosensitive layer 4 formed on the rough substrate surface 3 is different from portion to portion, for example, as shown in T21 or T22 of FIG. 15B, and therefore although the light beams L21, L22, and L23 reflected irregularly on the substrate surface 3 are reflected regularly on the surface of the photosensitive layer 4, their phases are different. Consequently, in the case where the substrate surface 3 is rough, no interference pattern due to the light beams L21, L22, and L23 is formed, so that it is prevented interference fringes from occurring in the images formed on the surface of the photoreceptor.
In general, the photosensitive layer of an electrophotographic photoreceptor is often formed by an immersing and coating method in which a substrate is immersed in a coating bath filled with a photoreceptor coating solution, and then the substrate is lifted at a predetermined rate, because of high productivity. In this immersing and coating method, when lifting the substrate, stripes are generated in the direction opposite to the lifting direction, so that non-uniformity in the thickness tends to be generated. In addition, an organic solvent that easily evaporates is contained in the coating solution, so that only the solvent evaporates from the coating solution in the coating bath and the viscosity and the concentration of the coating solution is changed. As a result, the thickness during coating is unstable.
For prevention of non-uniform thickness and stable formation of uniform thickness, the thickness of the layer is measured with a high precision in the course of coating and forming the photosensitive layer on a substrate, and the amount of coating is controlled in accordance with the measurement results so as to adjust the thickness. For this purpose, various methods for measuring the thickness of the photosensitive layer are proposed. As the methods for measuring the thickness, contact methods for measuring a film thickness with a step height meter, an eddy current meter for measuring a film thickness or the like, and non-contact methods for measuring a film thickness such as a color and color-difference method, optical interferometry, and an optical absorption method are used, but optical interferometry is most commonly used because operation is comparatively simple, and measurement can be performed in a short time (e.g., see Japanese Unexamined Patent Publication JP-A 4-336540 (1992, page 4, FIG. 2)).
Hereinafter, the principle on which the thickness of a layer is measured by optical interferometry will be described briefly. FIGS. 16A and 16B are views showing reflection behavior of light in transparent film 5 and 7, respectively. FIG. 16A shows the manner in which a light beam L31 incident to the transparent film 5 is multiple-reflected in the transparent film 5. The light measured as a reflected light beam L32 from a surface 5a of the transparent film 5 is a light beam obtained by synthesizing light beams that are multiple-reflected in the transparent film 5. Light is a wave, so that when synthesizing light beams, if a phase difference is an integer multiple of 2 π, the light beams are mutually intensified, and if a phase difference is an odd integer multiple of π, the light beams are canceled each other and interference occurs.
FIG. 16B shows the manner in which light is reflected in the transparent film 7 formed on the substrate 6. The reflectance R of the light in the transparent film 7 formed on the substrate 6 can be obtained based on Equation (1):Reflectance R={R12+R22−2R1R2 cos(X)}/{1+R12+R22−2R1R2 cos(X)}  (1)where X=4 πN1d/λ
λ: wavelength of light
d: thickness of a transparent film
R1: reflectance on a surface of a transparent film
R2: reflectance on a surface of a substrate
N1: refractive index of a transparent film
N2: refractive index of a substrate
where N2>N1.
The reflectance R1 in the surface 7a of the transparent film and the reflectance R2 in the surface 6a of the substrate can be obtained based on Equations (2) and (3), respectively.R1=(1−N1)/(1+N1)  (2)R2=(N1−N2)/(N1+N2)  (3)
The reflectance R becomes the largest value (or the smallest value) in a wavelength with which light beams are mutually intensified (or weakened) by optical interference, so that when the reflectance R is differentiated with a wavelength λ to obtain a wavelength that provides the largest (or the smallest) reflectance R, Equation (4) can be obtained.(1/λn)−(1/λn+1)=½N1d  (4)where λn: a wavelength having the nth largest value (or smallest value).
When the wavelength with which light beams are mutually intensified (or weakened) and the refractive index are known, the thickness d of the transparent film 7 can be obtained based on Equation (4). The refractive index of the film and the wavelength can be measured with, for example, a spectrophotometer, and therefore the thickness of the film can be obtained with the measurement results based on the Equation (4). For a film whose refractive index is not known, a film having a defined thickness is formed and the refractive index of the film whose thickness is known is obtained based on Equation (4) in advance, so that an arbitrary thickness of a film formed of the same material can be obtained.
Thus, the optical interferometry measures the thickness of a photosensitive layer utilizing an interference pattern of light that is multiple-reflected in the photosensitive layer of an electrophotographic photoreceptor. Therefore, when the surface of the substrate of the electrophotographic photoreceptor is made rough to prevent interference fringes that cause non-uniformity in the images so as to weaken the interference based on reflection on the substrate surface and the surface of the photosensitive layer, it becomes difficult to measure the thickness of the photosensitive layer.
In order to solve such a problem, light having a wavelength longer than a surface roughness of the substrate shown in the ten-point average roughness (Rz) defined in Japanese Industrial Standard (JIS) B0601 is used as the light used for measuring the thickness of the photosensitive layer to suppress disappearance of the peak during synthesis of light beams so that the thickness can be measured even with weak interference (e.g., Japanese Unexamined Patent Publication JP-A 2000-356859 (2000, page 4, FIG. 6).
However, the technique disclosed in JP-A 2000-356859 also has the following problem. With higher resolution of an image forming apparatus, the spot diameter of light for writing electrostatic latent images on the surface of the electrophotographic photoreceptor has been increasingly reduced. When the spot diameter of light is reduced, the interference fringes may occur, regardless of the rough surface of the substrate of the electrophotographic photoreceptor. Therefore, when the spot diameter of light is small, the surface roughness of the substrate tends to be made rougher in order to prevent interference fringes from occurring, and light having a longer wavelength is used as the light used for measuring the thickness as the surface roughness becomes rougher. Thus, when the wavelength of light used for measuring the thickness becomes longer, the distance between adjacent wavelengths is increased, so that the measurement precision of the thickness is reduced, or the measurement cannot be performed.